This is a Continuation Application of PCT Application No. PCT/JP02/06641, filed on Jul. 1, 2002, which was not published under PCT Article 21(2) in English. This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-198916, filed on Jun. 29, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a hollow structure with a flange, and more particularly, to a hollow structure with an annular flange having an improved durability.
2. Description of Related Art
A hollow structure with an annular flange attached therearound which has a temperature gradient in the direction of the thickness of the wall of the hollow structure during operation has been utilized in various technical fields.
For example, a conventional gas turbine system includes a plurality of combustors for supplying a working gas, i.e. combustion gas into a gas turbine. Such a plurality of combustors are circumferentially disposed around a central axis of a turbine rotor in an adjacent relationship to each other. Each of the combustors comprises a cylindrical structure including a liner with a combustion nozzle attached at one end thereof and a tail cylinder connected to the liner. The tail cylinder includes an end annular flange at the outer circumference of a discharge opening thereof in order to connect it to a turbine casing.
According to the configuration of the prior art combustion system described above, combustion gas is injected into the inner space of the combustor via one or more combustion nozzles, the combustion gas will be mixed with the air in the inner space, and this mixture then flows longitudinally toward the discharge opening of the combustor, the mixture thereafter being supplied to a turbine rotor.
One drawback of such a prior art combustion system, however, arises due to the fact that a temperature gradient occurs at the end annular flange in the direction of the thickness of the wall of the tail cylinder caused by a difference between the temperature of the inner space and that of the outer environment thereof.
When a gas turbine system is in its normal operating state, the temperature of the inner surface of the tail cylinder will rise significantly due to it being exposed to the combustion gas having a maximum temperature of 1500xc2x0 C., while the temperature of the outer surface of the tail cylinder will not rise very much since it is cooled by the outer environmental ambient air, so that there may occur temperature gradient of a several hundreds of degrees in the direction of the thickness of the wall of the tail cylinder. Therefore, if the end flange had a discharge opening rectangular in shape, the discharge opening would be deformed due to the wide range of temperature gradient described above, so that long wall sides of the end flange would be bent inwardly while short wall sides thereof would be bent outwardly, resulting in inducing a high level of thermal stress at each of the corners therebetween, thereby possibly creating cracking therein by thermal fatigue.
In order to solve this problem, an apparatus has been proposed which can reduce the range of the temperature gradient in the direction of the thickness of the wall of the tail cylinder. More particularly, the apparatus has a cooling means comprising a flow passage disposed near the inner surface of the tail cylinder to provide a flow path for a coolant fluid therethrough, thereby lowering the temperature of the inner surface of the tail cylinder.
This apparatus, however, has an inherent drawback.
That is, if the temperature of the inner surface and, therefore, the inner space of the tail cylinder is lowered, the efficiency of the gas turbine system will deteriorate.
Another type of a known tubular-shaped hollow structure, such as an exhaust duct, has a plurality of annular flanges fixedly attached to the outer circumference thereof in an equally spaced apart longitudinal relationship with each other. When exhaust gas having a high temperature flows through the exhaust duct, the hollow structure, i.e. the exhaust duct, expands outwardly due to the wide range of a temperature gradient in the direction of the thickness of the wall of the duct, similar to the case of the combustor described above. At this time, since the thermal expansion of the outer surface of the exhaust duct is restricted by each of the plurality of the flanges, the hollow structure is deformed into a wave-like shape in a longitudinal direction thereof, so that each of the portions of the hollow structure at which each of the plurality of the flanges are attached form sinks of the wave, while other portions of the hollow structure located between two adjacent flanges will form peaks of the wave, thereby causing material of which the wall of the hollow structure consists being bent. Thus, the hollow structure will undergo a high level of a thermal stress at the portions where the plurality of the flanges are respectively attached thereto, as in the case of the combustor described above.
It is an object of the invention to provide a gas turbine combustor in which the thermal stress around the end annular flanges thereof can be minimized during normal operation without damaging the efficiency of the gas turbine system.
Another object of the invention is to provide a hollow structure having at least one flange which can eliminate the possibility of crackings being created due to the thermal fatigue, thereby improving the durability thereof.
The factors which define the thermal expansion of a material generally include a thermal expansion coefficient of the material, an amount of a temperature difference, and a initial length of the material. In the art, the temperature difference has been mainly controlled by cooling the prior high temperature portion of the material at a lower maximum temperature so as to reduce the thermal stress of the end flange of the combustor during normal operation, resulting in damaging the efficiency of the gas turbine system, whereas the present invention, which is based upon controlling the thermal expansion coefficient of the material in order to avoid damaging the efficiency of the gas turbine system, utilizes a plurality of materials having coefficients of linear expansion different from each other at around the end annular flange, thereby reducing the amount of the thermal stress.
The present invention relates to a hollow structure with a flange wherein at least one annular flange is fixedly attached around the hollow structure which has a temperature gradient in the direction of the thickness of the wall thereof comprising said at least one annular flange including an outer portion formed of a metal material having a coefficient of linear expansion greater than that of a metal material different from the former one forming another portion of said at least one annular flange.
In accordance with one aspect of the present invention, by forming a portion of the flange exposed to a lower temperature from a metal material having a coefficient of linear expansion greater than that of a metal material different from the former one forming a portion of the flange exposed to a higher temperature, the difference between the thermal expansion of the higher temperature portion and that of the lower temperature portion will become less due to the larger amount of expansion of the lower temperature portion (so-called bimetal effect) during normal operation, than would be in the case of the higher and lower temperature portions of same materials, resulting in restriction of the hollow structure from being expanded radially outwardly is reduced such that there is less deforming of the hollow structure into a wave-like shape in longitudinal direction thereof, thereby reducing the thermal stress at the portions where each of the plurality of the flanges are attached thereto so as to eliminate the possibility of creating crackings due to the thermal fatigue.
Therefore, in one preferred embodiment of the present invention, a hollow structure is one of a plurality of gas turbine combustors disposed circumferentially around a central axis of a turbine rotor in an adjacent relationship to each other which one of a plurality of gas turbine combustors includes a generally tubular-shaped tail cylinder for providing a flow path for combustion gas flowing longitudinally toward the discharge opening thereof, said tail cylinder having an end annular flange at the outer circumference of the discharge opening thereof for connecting it to a turbine casing, wherein the end annular flange comprises said at least one flange; wherein said end annular flange includes an outer portion formed of a metal material having a coefficient of linear expansion greater than that of a metal material forming another portion of said end annular flange.
In accordance with this aspect of the present invention, by forming a portion exposed to a lower temperature from a metal material having a coefficient of linear expansion greater than that of a metal material different from the former one forming a portion exposed to a higher temperature, the difference between the thermal expansion of the higher temperature portion and that of the lower temperature portion will become less due to the larger amount of expansion of the lower temperature portion (so-called bimetal effect) during normal operation, than would be in the case of higher and lower temperature portions of the same materials, resulting in a reduced amount of bending deformation, thereby reducing the thermal stress due to the deformation, especially at the corner portions thereof, so as to eliminate the possibility of creating crackings due to the thermal fatigue.
Alternatively, the thermal stress at the corner of the discharge opening can be reduced by providing a bimetal structure along either long sides or short sides of this opening.
In accordance with a further aspect of the present invention, said end annular flange includes a first annular portion formed of a first metal material forming a circumferential portion of the discharge opening, and a second annular portion formed of a second metal material having a coefficient of linear expansion greater than that of the first metal material; wherein said first and second annular portions are joined to each other by welding with a flux having a coefficient of linear expansion between that of the first metal material and that of the second metal material.
Preferably, the coefficient of linear expansion of the second metal material is 10% greater than the coefficient of linear expansion of the first metal material.
More preferably, the metal material forming the inner portion of said flange is hastelloy-X, and the metal material forming the outer portion of said flange is A-286 or SUS stainless steel.